1. Field of the Invention
The present invention relates to a chromatic dispersion compensating module which compensates the chromatic dispersion of an optical fiber transmission line, and an optical transmission system using the same.
2. Related Background Art
Optical communication using an optical fiber transmission line enables a high-speed and large-capacity information transmission. The bit rate in optical transmission is increasing from 10 Gb/s to 20 Gb/s, and furthermore, to 40 Gb/s. In such a high-speed optical transmission, the waveform degradation of each signal caused by the chromatic dispersion intrinsic to the optical fiber transmission line is found problematic. More specifically, with increasing bit rate, a further minimized absolute value of cumulative chromatic dispersion is required for the optical fiber transmission line set between the transmitting end and the receiving end through which the signals propagate. Accordingly, there is employed a dispersion-shifted optical fiber having a zero-dispersion wavelength in a signal wavelength band (i.e., the 1.55 xcexcm wavelength band) as the optical fiber transmission line which enables a signal transmission with small waveform degradation at a high bit rate.
However, the optical fiber transmission line is generally installed on land or undersea, i.e., under natural environments. Thus, the absolute value of chromatic dispersion is subject to natural conditions, and may be varied due to factors such as the diurnal or seasonal temperature fluctuation. In such a case, even if the dispersion-shifted optical fiber should be employed for the optical fiber transmission line, the waveform degradation of each signal propagating through the optical fiber transmission line is not negligible, and there may occur cases in which a signal transmission at a high bit rate is unfeasible.
As a technology for overcoming the above-mentioned problem is disclosed, for example, in document 1, Kuwahara et al., xe2x80x9cStudy on optimized dispersion equalizing method by detecting dispersion fluctuation using PM-AM conversion effectxe2x80x9d, Proc. of Electronic Information Communication Society, 1998, B-10-95 (1998), or in document 2, Ooi et al., xe2x80x9cAutomatic dispersion equalizing experiment on 40 Gbit/s transmission using wavelength variable lasersxe2x80x9d, Proc. of Electronic Information Communication Society, 1998, B-10-96 (1998).
In the technology disclosed in the documents 1 and 2, the light (signals) reached to the receiving end of the optical fiber transmission line is monitored to measure the cumulative chromatic dispersion or the change in cumulative chromatic dispersion of the optical fiber transmission line. The wavelength of each output signal is then controlled on the transmitting end based on the thus acknowledged measured results. More specifically, the wavelength of each signal outputted from the transmitting end is controlled in such a manner that the measured cumulative chromatic dispersion of the optical fiber transmission line should be minimized at the receiving end. In this manner, signals having a wavelength in the vicinity of zero-dispersion wavelength constantly propagate through the optical fiber transmission line. As a result, the waveform degradation of each signal is suppressed to enable a signal transmission of high bit rate.
The present inventors studied the above-mentioned prior art technology, and have found problems as follows. That is, in the constitution of the technology disclosed in the documents 1 and 2, the measurement results of the cumulative chromatic dispersion of the optical fiber transmission line are acknowledged to the transmitting end. This implies that the constitution further requires additional communication means and signal transmission lines. Furthermore, to control the wavelengths of the signals emitted from the transmitting end in accordance with the acknowledged results, there must be provided a light source with wavelength changeable function. Thus, the technology disclosed in the documents 1 and 2 inevitably requires an optical transmission system with a complicated constitution.
The present invention has been made with an aim to overcome the aforementioned problems. Thus, an object of the present invention is to provide a chromatic dispersion compensating module having its simple constitution, and yet capable of realizing a signal transmission of high bit rate by compensating for the chromatic dispersion of the optical fiber transmission line, and to provide an optical transmission system using the same.
The optical transmission system according to the present invention is applicable to WDM (Wavelength Division Multiplexing) communication using signals having a plurality of wavelengths propagating through one or more optical fiber transmission lines. The optical fiber transmission lines can be provided at least between the transmitter and the receiver, between the transmitter and a repeater, between repeaters, or between a repeater and the receiver.
The chromatic dispersion compensating module according to the present invention, which is applicable to the optical transmission system, has a function of compensating for the chromatic dispersion of the optical fiber transmission line, and comprises at least a chromatic dispersion compensator and a temperature controller. The chromatic dispersion compensator has, at a predetermined wavelength, a chromatic dispersion with a sign opposite to that of the optical fiber transmission line that is the object to be compensated, and a chromatic dispersion slope with a sign opposite to that of the optical fiber transmission line. The temperature controller sets the chromatic dispersion of the chromatic dispersion compensator at a desired value by controlling the temperature of the chromatic dispersion compensator.
In accordance with a chromatic dispersion compensating module having the above-mentioned structure, at a predetermined wavelength of, for example, 1550 nm, the cumulative chromatic dispersion as viewed from the entire optical transmission system can be effectively reduced because the signs of the chromatic dispersion of the optical fiber transmission line and the chromatic dispersion compensator are set opposite to each other. In case the optical transmission system is constructed from a plurality of optical fiber transmission lines via one or more repeaters, it is preferable that the chromatic dispersion compensator is prepared to each of the optical fiber transmission lines (such that the chromatic dispersion compensators are each set on the repeaters and the receiver). Furthermore, since the signs of the chromatic dispersion slopes of the optical fiber transmission line and the chromatic dispersion compensator are set opposite to each other, the cumulative chromatic dispersion as viewed from the entire optical transmission system for each of signals having a plurality of wavelengths can be effectively reduced over a wavelength broader band. In addition, even if the chromatic dispersion of the optical fiber transmission line should change attributed to a temperature fluctuation and the like, the temperature of the chromatic dispersion compensator is controlled by the temperature controller. Accordingly, in this case again, the cumulative chromatic dispersion as viewed from the entire optical transmission system can be effectively reduced.
Further, in the chromatic dispersion compensating module according to the present invention, the chromatic dispersion compensator preferably includes a dispersion compensating optical fiber. The dispersion compensating optical fiber constitutes a part of the transmission line of the optical transmission system, and it minimizes the insertion loss. Moreover, in the chromatic dispersion compensating module according to the present invention, at a predetermined wavelength, the absolute value of the temperature dependence in the chromatic dispersion of the dispersion compensating optical fiber, i.e., the absolute value in the amount of chromatic dispersion fluctuation per unit temperature, is preferably larger than the absolute value of chromatic dispersion fluctuation per unit temperature for the optical fiber transmission line. Otherwise, at a predetermined wavelength, the absolute value of temperature dependence in chromatic dispersion for the dispersion compensating optical fiber is preferably 0.002 ps/nm/km/xc2x0 C. or higher. In either cases, an efficient chromatic dispersion compensation is possible because the temperature of the dispersion compensating optical fiber is properly controlled.
The chromatic dispersion compensating module according to the present invention, furthermore, may comprise additionally a chromatic dispersion controlling unit which controls the temperature controller (i.e., for temperature control of the chromatic dispersion compensator), to thereby control the chromatic dispersion of the chromatic dispersion compensator. By thus providing the chromatic dispersion controlling unit, the temperature control, which is performed by the temperature controller, of the chromatic dispersion compensator is performed to control the chromatic dispersion of the chromatic dispersion compensator in such a manner that the cumulative chromatic dispersion as viewed from the entire section to be compensated by the chromatic dispersion compensator, the section including the optical fiber transmission line and the chromatic dispersion compensator, can be maintained substantially zero.
The chromatic dispersion compensating module according to the present invention may further comprise, in addition to the chromatic dispersion controlling unit which controls the temperature controller, a chromatic dispersion measuring unit which measures the cumulative chromatic dispersion or the change in cumulative chromatic dispersion of the optical fiber transmission line by monitoring light inputted into the chromatic dispersion compensator. In this case, a feed-forward control is applied to the chromatic dispersion of the chromatic dispersion compensator in such a manner that the cumulative chromatic dispersion as viewed from the entire section to be compensated by the chromatic dispersion compensator, the section including the optical fiber transmission line and the chromatic dispersion compensator, is maintained substantially zero.
Furthermore, the chromatic dispersion compensating module according to the present invention may comprise, in addition to the chromatic dispersion controlling unit which controls the temperature controller, a chromatic dispersion measuring unit which monitors the signal output from the chromatic dispersion compensator and thereby measures the cumulative chromatic dispersion or the change in cumulative chromatic dispersion of the entire section to be compensated by the chromatic dispersion compensator, the section including the optical fiber transmission line and the chromatic dispersion compensator. In this case, a feed-back control is applied to the chromatic dispersion of the chromatic dispersion compensator in such a manner that the cumulative chromatic dispersion as viewed from the entire section to be compensated by the chromatic dispersion compensator, the section including the optical fiber transmission line and the chromatic dispersion compensator, is maintained substantially zero.
If the amount of chromatic dispersion or the temperature dependence of the chromatic dispersion that is to be controlled for the temperature fluctuation is already known, the value of chromatic dispersion need not be monitored. In this case, the chromatic dispersion compensating module according to the present invention determines the temperature to be set based on the amount of chromatic dispersion necessary to be controlled and the temperature dependence of the chromatic dispersion, and hence, the chromatic dispersion compensating module may additionally comprise a structure which controls the temperature of the chromatic dispersion compensator while monitoring the temperature of the chromatic dispersion compensator. More specifically, the chromatic dispersion compensating module may further comprise, in addition to the chromatic dispersion controlling unit which controls the temperature control unit, a temperature measuring unit which predicts the cumulative chromatic dispersion or the change in cumulative chromatic dispersion as viewed from the entire section to be compensated by the chromatic dispersion compensator, the section including the optical fiber transmission line and the chromatic dispersion compensator.
On the other hand, the optical transmission system according to the present invention comprises an optical fiber transmission line and a chromatic dispersion compensating module as described above, which compensates for the chromatic dispersion of the optical fiber transmission line. In accordance with the optical transmission system, at a predetermined wavelength of, for example, 1550 nm, the chromatic dispersion of the optical fiber transmission line can be compensated by the chromatic dispersion compensating module. Furthermore, even if the chromatic dispersion of the optical fiber transmission line should fluctuate due to some factors such as a temperature fluctuation, the temperature of the chromatic dispersion compensator is properly controlled by the temperature controller. In this manner, the cumulative chromatic dispersion as viewed from the entire optical transmission system or the entire section to be compensated by the chromatic dispersion compensator can be reduced as to enable a signal transmission at a high bit rate.
The chromatic dispersion compensating module applied to the optical transmission system according to the present invention is preferably installed at the down stream side of the optical fiber transmission line to be compensated, i.e., at a position located between the light output end of the optical fiber transmission line and the receiver. Furthermore, since the compensation amount of the chromatic dispersion is optimally controlled in case the chromatic dispersion compensating module comprises the chromatic dispersion measuring unit and the chromatic dispersion controlling unit, the signal transmission can be performed constantly at a high bit rate.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.